Aluminum metaphosphate optical fibers

ABSTRACT

Aluminum metaphosphate optical fibers are disclosed. In a specific embodiment, aluminum metaphosphate, doped with from 10 to 30 mole percent of diboron trioxide, is found to yield an optical fiber which combines the desirable properties of both high numerical aperture and low material dispersion. The fiber is nonhygroscopic and has a high melting temperature. The index of refraction of the glass may be lowered by doping with silicon dioxide. Consequently, a graded fiber may be made by increasing the concentration of silicon oxide from the core to the cladding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention involves optical fibers.

2. Description of the Prior Art

Optical fiber fabrication technology has advanced sufficiently far sothat at the present time sophisticated engineering designs can bereduced to specific embodiments utilizing available fabricationprocesses. Both single mode and multimode fibers of very low loss arecurrently fabricated as a matter of course. In addition, mode dispersionin multimode fibers can be minimized by fabricating fibers with aradially graded index of refraction.

Despite the large strides that have been made in optical fiberfabrication technology, certain design parameters still remainunrealized because of limitations in either the fabrication process ofthe properties of the material used. One such area in which significanteffort is still being applied involves the fabrication of optical fiberswith high numerical aperture (N.A.). Such fibers require a core whoseindex of refraction is significantly greater than that of the cladding.However, materials with high enough index of refraction to be used inhigh N.A. fibers are most often found to have deleterious shorcomings inother areas; for example, they cannot be easily graded; they arehygroscopic; they melt at low temperatures; or they display highmaterial dispersion.

SUMMARY OF THE INVENTION

This invention is an aluminum metaphosphate based optical fiber. Thefiber comprises a core of aluminum metaphosphate, which, in a specificembodiment, is doped with from 10 to 30 mole percent diboron trioxide.An exemplary cladding consists of aluminum metaphosphate diborontrioxidedoped with silicon dioxide. The fiber of such an embodiment has highnumerical aperture, low material dispersion and high chemicaldurability, allowing for the transmission of high optical powers at highbandwidths. Fibers with numerical aperture of 0.48, and materialdispersion lower than that measured in SiO₂ have been fabricated usingsuch materials.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of the inventive fiber.

FIG. 2 is a compositional diagram indicating compositions which may beutilized in the inventive fiber.

FIG. 3 shows the behavior of the index of refraction of the diborontrioxide doped aluminum metaphosphate glass as silicon dioxide is addedto the glass.

FIG. 4 is a plot of the refractive index of the glass versus wavelengthfor various inventive compositions.

FIG. 5 is a plot of the material dispersion of the glass as a functionof wavelength for various inventive compositions.

FIG. 6 is a plot of the first derivative of the index of refraction as afunction of a wavelength for various inventive compositions.

FIG. 7 is a plot of alpha, related to the optimum grading profile, as afunction of wavelength, for an inventive composition as well as for aprior art fiber.

DETAILED DESCRIPTION OF THE DRAWING

The invention is an optical fiber based upon the ternary system aluminummetaphosphate diboron tiroxide silicon dioxide. (The variouscompositions may vary from perfect stoichiometry, and variations of asmuch as 5 molar percent from perfect stoichiometry, if not more, arecontemplated within the scope of this invention.) The glass is found tohave a high index of refraction, while at the same time having lowmaterial dispersion. Higher concentrations of silicon dioxide lower theindex of refraction of the glass. High numerical aperture fibers, i.e.,with relatively large differences in index of refraction between thecore and the cladding, may be made from this glass by fabricating a corethat has low or zero concentration of silicon oxide and a cladding withhigher concentrations of silicon oxide.

The properties of the inventive fiber can only be appreciated with anunderstanding of the effects that dispersion and refraction have on afiber's transmission capability.

The refractive properties of a fiber determine, in the first instance,whether it will guide light, as well as its numerical aperture. Thenumerical aperture of the fiber, in turn, determines the solid angleover which the fiber can effectively receive light and transmit it. Thenumerical aperture varies directly with the difference in index ofrefraction between the core and the cladding. Consequently, infabricating fibers with high numerical aperture, glass compositionswhich can be fabricated with large variations in index of refraction aredesirable. The index of refraction of the composition is altered, eitherraised or lowered, by changing the amount of one or more constituents ofthe composition. However, under most circumstances, increased index ofrefraction, required for high N. A. fibers, is associated withdeleteriously high material dispersion, in part because the addedelements, necessry to increase the index, complicate the interactionbetween the material and the light.

The dispersive characteristics of a fiber are critical to its ultimateinformation carrying capacity. Transmission of an optical pulse througha highly dispersive fiber will result in significant broadening in thetemporal extent of the pulse. Consequently, only a limited number ofpulses could be transmitted along such a fiber without pulsesoverlapping, thereby limiting the information capacity of the fiber.Dispersive effects can be traced to material limitations--under whichconditions they are referred to as material dispersion--or to requiredcompositional variations in multimode fibers--under which conditionsthey are referred to as mode dispersion.

The material dispersion of a fiber must be carefully distinguished fromthe mode dispersion associated with a multimode fiber. The modedispersion of a multimode fiber is due to the fact that the light istransmitted at different velocities depending upon the mode in which itis transmitted. The velocity of each mode is determined by the spatialconfiguration of the mode and the composition of the fiber in thatregion where the mode is prevalent. Material dispersion, however, is dueto the fact that the index of refraction of the glass is different fordifferent wavelengths. A pulse of light transmitted through the fibermay be viewed mathematically as the sum of a Fourier series of a numberof different wavelengths of light. If the index of refraction and hencethe velocity with which the light is transmitted through the fiber isdifferent for different wavelengths, then the different Fouriercomponents of the pulse will be transmitted at different velocities.Hence, with time, the pulse will lose its temporal definition and willbroaden.

It may be seen from the above discussion that the material dispersionwill be minimized at that point at which the index of refraction isapproximately independent of wavelength. For optical fibers, this occursapproximately midway between the U.V. electronic absorption peaks andthe infrared vibrational absorption peaks. The low dispersion of theinventive fiber discussed here is believed due to the relatively lowatomic weight of the aluminum and boron consitituents of the glass.These low masses shift the vibrational constant of the associatedmolecular system towards the U.V. Consequently, the point of zeromaterial dispersion also shifts to shorter wavelengths and appears nearthe visible region, of interest in optical fiber application.

The inventive fiber is shown schematically in FIG. 1, which, in anexemplary embodiment, has a radially graded index of refraction. Thefiber, 13, comprises a core, 11. The core comprises aluminummetaphosphate doped, in an exemplary embodiment, with from 10 to 30percent diboron trioxide. The cladding consists of a glass with a lowerindex of refraction. Such exemplary glasses include silicon dioxide,diboron trioxide or silicon dioxide doped with from 10 to 20 molepercent diboron trioxide, all of which are found to have lower indicesof refraction than the core material described above. While any of theabove mentioned glasses can be used as a cladding, the fabricationprocess will be significantly simplified if the cladding comprises thecore material doped with sufficient silicon dioxide to lower the indexof refraction to a predetermined required value. A specific embodimentthen comprises a core of high index of refraction and a cladding of alow index of refraction, but both with similar constituents. The coreand/or the cladding may be graded in the radial direction to lower themode dispersion. Such grading is most easily accomplished with theinventive composition by adding additional silicon dioxide to the borondoped aluminum metaphosphate, as discussed above.

In a graded multimode fiber, the cladding may have less clearly definedspatial boundaries than in a single mode fiber. In such multimodefibers, grading may extend from the center of the "core" to theperiphery of the cladding with or without the addition of new elementsat various points along the radius. In the inventive fiber, for example,the gradation may extend from a ternary core to pure borosilicate at theperiphery of the "claddng."

An advantage that accures from the use of the herein disclosedcomposition in graded fibers involves the relatively small variation incoefficient of expansion as the composition is varied. In prior artfibers, such as for example, those of germanium silicate, high N. A.configurations may result in a coefficient of expansion which varies bya factor of ten across the fiber. Other fibers such as those made ofgermania borosilicate may have lower coefficient of expansionvariations, but cannot attain the N. A.'s obtained in the fibersdisclosed here. The inventive fibers disclosed here may have acoefficient of expansion which varies by as little as a factor of two asthe composition is varied to grade the fiber.

The inventive glass compositions are shown schematically in FIG. 2. Inthis Figure, the amount of aluminum metaphosphate, diboron trioxide andsilicon dioxide in the glass are shown in a triangular plot. Sixspecific compositions on which extensive measurements were made areindicated by the letters "A" through "F". As shown in the Figure, anexemplary inventive fiber comprises aluminum metaphosphate doped withfrom 10 to 30 mole percent diboron trioxide. Alternative embodimentscomprise aluminum metaphosphate doped with from 15 to 25 mole percentdiboron trioxide. The index of refraction of the glass may be lowered byadding silicon dioxide and this ternary system is also displayed in thefigure. As additional silicon oxide is added, it is found advantageousto lower the amount of boron oxide. FIG. 3 shows how the index ofrefraction of the aluminum metaphosphate diboron trioxide system may belowered by adding silicon dioxide to the glass.

The compositions which form the data points of FIGS. 2 and 3 may beformed by mixing reagent grade powdered silicon oxide, boron oxide andaluminum metaphosphate in acetone while stirring to prevent preferentialseparating of the powder. The mixture is dried overnight at 200 degreesC. and is then prefired at 1400 degrees C. for half an hour in aniridium crucible. The resultant material is placed in a furnace at 1600degrees in an air atmosphere and melted. The material is stirred severaltimes during melting and firing to ensure homogeneity. Subsequent to themelting, the material is poured in a stainless steel mold and slowlycooled to room temperature. The index of refraction and materialdispersion measurements are made using techniques well known in the art.

FIG. 4 is a plot of the refractive index of the various compositionsdescribed in FIGS. 2 and 3 as a function of wavelength. The fact thatcomposition E has an index of refraction lower than that of pure silicamay be attributed to the index affecting properties of the boron dopant.This effect forms the subject of a commonly assigned patent (U.S. Pat.No. 3,778,132).

FIG. 5 is a plot of the material dispersion of the various glasscompositions of FIGS. 2-4 as a function of wavelength. A specificinventive embodiment involves a fiber with both low material dispersionand high numerical aperture. For example, a fiber with a core ofcomposition A and a cladding of composition E will have a numericalaperture of 0.48, whereas a typical prior art fiber with a core ofcomposition G and a cladding of composition E will have a numericalaperture of 0.22. Consequently, the inventive higher N.A. fiber is seento have a lower material dispersion than a lower N.A. prior art fiber.The specific embodiment simultaneously combines the advantageouscharacteristics of high N.A. and low material dispersion. Specificembodiments include fibers with N.A.'s greater than 0.3 and materialdispersion less than 0.07 nanoseconds/nanometer-kilometer at 0.9 micronsas well as fibers with N.A.'s between 0.3 and 0.5 and materialdispersion less than 0.07 nanoseconds/nanometer-kilometer at 0.9microns.

The significance of low material dispersion has recently becomeheightened in view of the fact that mode dispersion in radially gradedfibers can now be lowered by appropriate grading to approximately 1-2nanoseconds per kilometer. In prior art germania-silica fibers, materialdispersion of 100 picoseconds per nanometer-kilometer at 0.9 micronsyield a 3 nanosecond per kilometer dispersion when a light-emittingdiode with a 30 nanometer bandwidth is used. Clearly, in such a case,the material dispersion is greater than the mode dispersion and becomesthe limiting factor in determining the fiber's bandwidth. As fibertechnology advances and even lower mode dispersions are attained, theneed for low material dispersion fibers will become even greater. Theinventive aluminum metaphosphate fibers have a material dispersion ofapproximately 60 picoseconds per nanometer-kilometer at 0.9 microns, asseen from FIG. 5, and consequently, for a light-emitting diode with a 30nanometer bandwidth, the inventive fiber has a material dispersionapproximately equal to the best attainable mode dispersion.Consequently, unlike the prior art fibers, the inventive fiber is notmaterial-dispersion limited. This advantage of the inventive fiber isstill further heightened when one compares it to prior art fibers withthe higher N.A.'s of the present fiber, but which have much worsematerial dispersion.

FIG. 6 is a plot of the first derivative of index of refraction as afunction of wavelength for the various inventive compositions. Thisparticular quantity is of interest, since it enters into the calculationof the optimum radial gradation, for minimizing mode dispersion,discussed in the prior art and governed by the widely referred toconstant alpha (See, e.g., U.S. Pat. No. 3,989,350). Since thefabrication of a graded fiber involves the use of many differentcompositions, each of whose index of refraction displays a differentwavelength dependence, alpha itself may vary with wavelength in priorart fibers. However, in the inventive compositions, since the firstderivative of index of refraction is approximately the same for allglass compositions, alpha is found to be independent of wavelength, asshown in FIG. 7. This is to be compared with composition A in FIG. 7,which is a germanium silica graded fiber with numerical aperture of 0.22and which is found to have an alpha which varies significantly withwavelength. As a result, the germanium silica fiber has minimized modedispersion only at a single wavelength, whereas the alpha of theinventive fiber may be designed to have an optimum value essentiallyindependent of wavelength.

EXAMPLE

The fiber fabrication process envisioned in this invention may beessentially identical to those used with previous materials. Thealuminum may be obtained from trimethylalumina. Although this materialis found to ignite when exposed to oxygen, it may be contained in anoxygen-free cylinder in liquid form, and extracted by bubbling an inertcarrier gas through the cylinder to vaporize the liquid. An alternativesupply of aluminum in vapor form may be obtained from aluminumtrichloride which sublimates from a condensed phase. The phosphorous isobtained, according to techniques well known in the art, from POCl₃. Theappropriate glass precursor vapors are then reacted by prior artprocesses, such as the exemplary MCVD process (U.S. Patent ApplicationSer. No. 828,617 now U.S. Pat. No. 4,217,027) or hydrolysis processes,to form an appropriate glass optical fiber preform which is then drawninto an optical fiber.

Alternate fabrication processes may take advantage of the older bulkglass fiber fabrication technology. So, for example, an aluminummetaphosphate rod may be fabricated from bulk glass and coated with anexemplary borosilicate cladding using the exemplary hydrolysis process.Alternatively, the preform may be made using the older rod and tubetechnique wherein a rod and tube of appropriate compositions are nestedand the tube is collapsed over the rod. In a third alternativefabrication process, bulk glass of appropriate compositions may beformed into a fiber using the known double crucible technique. Theformation of the bulk glass may proceed as discussed in conjunction withthe compositions shown in FIGS. 2-4.

What is claimed is:
 1. An optical fiber comprising a core and acladding, the said fiber comprising aluminum metaphosphate, and in whichfiber the molar ratio of aluminum to phosphorus is given substantiallyby the formula Al(PO₃)₃.
 2. The fiber of claim 1 wherein the core isdoped with from 10 to 30 molar percent diboron trioxide.
 3. The fiber ofclaim 1 wherein the core is further doped with silicon dioxide.
 4. Thefiber of claim 2 or 3 wherein the index of refraction of the core isradially graded.
 5. The fiber of claims 1, 2 or 3 further comprising acladding comprising silicon dioxide.
 6. The fiber of claim 5 wherein thecladding futher comprises diboron trioxide.
 7. The fiber of claim 5wherein the cladding further comprises aluminum metaphosphate.
 8. Thefiber of claims 1, 2 or 3 further comprising a cladding comprisingdiboron trioxide.
 9. The fiber of claims 1, 2 or 3 further comprising acladding comprising aluminum metaphosphate, diboron trioxide and silicondioxide.